Saturday, 18 April 2009

The evolution of lethal intergroup violence - Raymond C. Kelly #

Department of Anthropology, University of Michigan, 550 East University Avenue, 101 West Hall, Ann Arbor, MI 48109-1092This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 20, 2004.Contributed by Raymond C. Kelly, July 14, 2005

Recent findings and analyses in evolutionary biology, archaeology,and ethnology provide a favorable conjuncture for examining theevolution of lethal intergroup violence among hominids during the2.9-million-year Paleolithic time span. Here, I seek to identify andinvestigate the main turning points in this evolutionary trajectoryand to delineate the periodization that follows from this inquiry.

collective violence armed conflict war hominid evolution

Detailed and well documented reports of intergroup attacksand killings among free-ranging chimpanzee groups wereinitially published during the period from 1979 to 1986 (1–4). Insubsequent years, data from additional research sites confirmedthat such episodes of lethal violence were characteristic ofchimpanzees as a species (5). These findings clearly raise animportant question with respect to the evolution of lethalintergroup violence: Did the same constellation of causal factorsthat gave rise to the chimpanzee pattern of coalitionary killingof neighbors produce a parallel (and convergent) outcomeamong Paleolithic hominids? Recent findings and analyses inevolutionary biology, archaeology, and ethnology create a favorableconjuncture for advancing our understanding of theevolution of lethal intergroup violence. These data providegrounds for evaluating Wrangham’s preliminary conclusion that‘‘current evidence supports the hypothesis that selection hasfavored a hunt and kill propensity in chimpanzees and humans,and that coalitional killing has a long history in the evolution ofboth species’’ (ref. 6, p. 1).The crux of Wrangham’s explanation of the evolution ofcoalitionary killing is that fitness is closely correlated with accessto food resources and that territorial enlargement thus enhancesfitness. In short, ‘‘fitness is correlated with territory size’’ (otherfactors being equal) (ref. 6, p. 15). Unprovoked attacks onmembers of a neighboring community thus convey a selectiveadvantage, provided that the costs to the attackers are low. Themechanism by which aggression is rewarded is intercommunitydominance. ‘‘If raiding leads to the wounding or death of aneighboring male, the neighboring community’s competitiveability is substantially reduced’’ (ref. 6, p.15). The dominantcommunity can thus freely encroach on the territory of itsneighbor whenever food resources within its own territory are inshort supply. The dominant community also may have anadvantage in recruiting reproductive females. However, thecapacity to translate additional females into increased fitnesswould be contingent on the commensurate expansion of foodresources.† Thus, although intercommunity dominance ‘‘tends tolead to increased fitness of the killers through improved accessto resources such as food, females, or safety’’ (ref. 6, p. 12),territorial gain is the critical ingredient for the realization of thisfitness enhancement.Wrangham applies his explanatory framework to an extensivearray of available data on intraspecific coalitionary killing ofadults among chimpanzees, bonobos, and wolves and is able toaccount for variability in the frequency of intercommunityattacks. For example, encounters between neighboring chimpanzeecommunities are most likely to eventuate in attacks whena single individual is spotted by a party of three or more maleswho are able to overwhelm a lone victim while incurring little riskof injury to themselves. Attacks are more frequent in thoseportions of the chimpanzee geographical range where competitionfor food resources is intensified by longer dry seasons.Lethal intercommunity attacks are not reported for bonobos,who rarely engage in the kind of dispersed feeding that exposeslone individuals to fatal attacks by neighbors (ref. 6, pp. 12–18).All in all, the available data largely support the imbalance-ofpowerhypothesis and the intercommunity dominance-drivehypothesis formulated by Wrangham.However, there is one apparent contradiction in Wrangham’sexplanatory framework. He documents border avoidance andunderutilization of food resources located in the border areasbetween group territories. At the Taı¨study site, for example, ‘‘. . .75% of time was spent in the central 35% of the [territorial]range’’ (ref. 6, p. 11). Although this substantial day-to-dayfrequency of border avoidance shows that coalitionary attacks byneighbors are anticipated (and constitute a pattern), such borderavoidance also entails a reduction in the effective size of groupterritories as repositories of food resources, because 65% ofthese resources cannot be exploited in complete safety.Available data are insufficient to precisely quantify the magnitudeof resource underutilization at all chimpanzee study sites.However, the Taı¨ data (cited above) clearly indicate that asubstantial percentage of the available food resources presentwithin a group territory are not exploited in this instance, andborder avoidance is established by Wrangham as a generalphenomenon.If fitness is enhanced by territorial enlargement, then fitnesswould be reduced by a pattern of lethal intercommunity attacksthat curtails resource availability along borders. Wranghamassesses the costs of aggression in terms of the risk of seriousinjury to the members of a party of chimpanzees carrying out anattack. Resource underutilization does not enter into his costbenefitcalculations or his fitness assessments. If the posited ‘‘. . .benefits to be gained by raiding or killing neighbors’’ (ref. 6, p.15) flow from territorial expansion that, in actuality, only addsa buffer zone, rather than food resources, then the value of thisbenefit would need to be recalculated. Factoring in a resourcereductionpenalty for initiating aggression, and rendering one’sown border zone unsafe as a result, also alters the cost side of theequation. The circumstances under which the potential benefitsof coalitionary attacks definitively outweigh the costs thusappear more narrowly circumscribed than Wrangham posits.Alternative adaptive strategies also may be superior in a broaderarray of contexts. Mutual nonaggression would clearly conveyboth mutual benefits and reduced costs. More specifically, thereported bonobo pattern of peaceful interactions between individualsof neighboring communities, in which ‘‘. . . they rest,travel, copulate, play, and groom together’’ (ref. 6, p. 17), entailsan absence of border avoidance that should convey a selectiveadvantage. The identification of a potential advantage linked tononaggression calls into question Wrangham’s conclusion thatselection favors lethal coalitionary attacks on neighbors and thatthese selective forces would produce a convergent evolutionaryoutcome among humans. As noted earlier, my principal concernis to assess the question of convergence.Recent analyses of intercommunity interactions among unsegmentedhunting and gathering societies‡ that are organizationallycomparable with those that existed during theUpper Paleolithic period (35,000 –10,000 B.P.) providegrounds for considering the applicability of the imbalance-ofpowerand intercommunity dominance-drive hypotheses (7).The Andaman Islanders (ca. 1858) constitute an especially aptcase in that resource scarcity is reported. In other words, thecondition of intensified food competition is present (as in theportion of the chimpanzee geographic range where intercommunityattacks are most frequent). Conflict between neighboringgroups over access to resources is more frequent amongthe Andamanese than among comparable hunting and gatheringsocieties in which resources are reportedly plentifulrelative to population (ref. 7, p. 142). The cooccurrence ofresource scarcity and conflict over resources is consistent withWrangham’s explanatory framework.However the cost–benefit relationship is very different fromthat described for chimpanzees, in that trespassing entails considerablerisk of injury. Intrusion into the territory of a neighboringgroup involves entering an area that is better known to itsowners than to the intruders. More importantly, the owners arehunters who stalk game with weapons that kill at a distance.There is no guarantee that intruders will spot members of theowning group before they themselves are spotted. Under thesecircumstances, an undetected lone hunter of the owning groupis not at a disadvantage in relation to a larger party of intruders.He may stalk them and kill one individual before fleeing to alertothers. Given superior knowledge of the territory, this course ofaction entails little or no risk.If two hunting parties become aware of each other’s presenceat the same time, the numerically smaller group withdraws andyields the contested food resource to the stronger party. However,if one party secures the advantage of remaining undetected,it employs this advantage to ambush the other party. The relativesize of the two parties is not a factor under these conditions ofasymmetrical detection. Moreover, substantial casualties areinflicted on these occasions. Thirteen encounters reported incolonial records (8) resulted in five serious wounds and sixdeaths, yielding an 85% overall casualty rate (ref. 7, p. 100). In1863, Andamanese guides thus cautioned early British colonialexploratory patrols that proceeding beyond a certain pointwould inevitably provoke a lethal attack.Jacko pointed to my heart and represented the act of asavage aiming at me with his bow and arrow, of the arrowpiercing my heart and my falling wounded, closing myeyes, and expiring. Topsy also pathetically enacted thedeath scene, and both waved their hands deprecatinglyin the direction disapproved of and entreated me not toproceed further but to return [to base].Ref. 8, p. 435This exploratory patrol was quite large in relation to Andamanesehunting parties. Firearms amplified this numerical advantage.Nevertheless, Jacko and Topsy clearly did not think thatthis superiority in numbers and weapons would render the risksinsignificant. It is implicit in their pantomime that trespassing isa capital offense. Intruders are presumed to be engaged in gametheft and are shot on sight.§ The first sign of contact withterritory owners is thus likely to be an arrow fired by an unseenarcher. In the region of most intense conflict, Jarawa men worebark torso armor extending from the armpits to the hips asprotection against being ambushed during the course of routinesubsistence activity (7).These ethnographic data show that the potential costs ofintrusion are extraordinarily high. The central premise ofWrangham’s imbalance-of-power hypothesis ‘‘ that one party canattack the other with impunity’’ (ref. 6, p. 1) is not met.Moreover, there is no realistic prospect for achieving intercommunitydominance that would enable one group to encroach onthe territory of a neighbor at will. The consequences of intergrouphostility are thus stalemate and an analogue of a war ofattrition in which there is a 0.02% mortality per annum fromspontaneous lethal conflicts over resources, with 85% of thismortality being among males (ref. 7, pp. 100 and 158). Suchconflicts are generally not sought but can occur when two groupsof hunters inadvertently encounter one another in the course ofthe food quest at the border of their respective territories.As noted among chimpanzees, the risk of encountering hostileneighbors encourages border avoidance. The concomitant underutilizationof food resources also reduces the effective size ofgroup territories. Thus, in the southern portion of the AndamanIslands, where endemic hostile intergroup relations obtain, thepopulation density (of 2.0 persons per square mile) is only 73%of that attained in North Andaman, where periodic peacepromotingjoint gatherings of neighboring bands entail mutualexploitation of food resources located in border areas. On theseoccasions, two bands (of about 45 individuals each) campedtogether at a site located in the zone between their respectiveterritories that might otherwise have been contested and thereforeavoided because of the possibility of ambush. The resourcesof this zone were exploited through joint hunting, fishing, andgathering expeditions, with the proceeds of these endeavorsshared and consumed at feasts. The general atmosphere of thesegatherings has been described as one of amiable rivalry in whichan effort is made to put aside residual ill feeling arising from pastquarrels. Gifts were exchanged, including bows, arrows, arrowheads,adzes, baskets, pigments, and shells for personal adornment.Each individual donor endeavored to outdo his or hercounterpart in generosity. In the course of the festivities, menperformed dances to the accompaniment of female singing (7).These gatherings provided a context for courtship leading tointermarriage. Kin ties between neighboring bands arising frommarriage were reinforced by reciprocal visiting and a culturalpractice of adoption whereby a child of one band was raised bya friend or relative of the parents residing in another band.¶ Thisrepertoire of peace-promoting practices facilitated more completeutilization of food resources in North Andaman, resultingin a higher population density (of 2.75 persons per square mile).If fitness is augmented by territorial enlargement (as a generalrule of selective advantage), then peace-promoting practices areadaptive and the maintenance of hostile intergroup relations ismaladaptive under conditions commensurate with those foundin the Andaman Islands. When relations with neighbors arefriendly, an average-sized territory of 16.4 square miles iscapable of supporting a band of 45 individuals. Hostile relationsshrink the utilizable area to the equivalent of 12 square miles,with a peripheral zone about one-third of a mile wide subject toborder avoidance. This reduced territory is sufficient to supporta band of only 33 individuals.When group territories are extensive and population densitiesare low, the percentage of territorial area rendered unexploitableby border avoidance is correspondingly reduced and the costof hostile relations with neighbors would be less than thatdocumented for the Andamanese. For example, a hunting andgathering band may require a territory of 100 square miles in aless favorable arctic or desert environment. Under these circumstances,avoidance of a one-third-mile-wide border zone wouldreduce a group’s effective territory by 11.4%. With a 200-squaremileterritory, the comparable figure is 8.1%. However, lowpopulation density also entails little or no competition for foodresources and infrequent encounters between members of neighboringgroups so that conflicts rarely occur and those that do arepeacefully resolved (ref. 7, pp. 133 and 142).The selective factors that give rise to coalitionary killing ofneighboring group members among chimpanzees do not have thesame effects in the case of unsegmented foraging societies.Fitness does correlate with territory size, or more accurately,effective territory size (provided that all proximate territoriesare occupied). Moreover, intergroup competition is intensifiedat higher population densities. But the attainment of intercommunitydominance is thwarted by a combination of factors thatincrease the costs to attackers. First, a party of hunters from onegroup cannot be certain that they are able to make an accurateassessment of the strength of the opposition. There is nocertainty that a single individual who has been sighted is indeedalone, because hunters often work in pairs or teams that stalkgame silently and employ ambush techniques. It is consequentlyunclear whether a favorable imbalance of power obtains. Thereis substantial risk that costs may be underestimated. Second,territory owners have intrinsic advantages over intruders that areakin to the advantages enjoyed by combatants who have set upan ambush in territory well known to them but not to opponents.Hunters lying in wait along game trails can readily kill intruders.Third, a hunter is a dangerous quarry because he possessesweapons that kill at a distance. Weapons augment the lethalityof combat and amplify the costs of assessment errors made byattackers. These errors include the potentially mistaken deductionsthat the party seeking to initiate an attack has remainedundetected (and enjoys the advantage of surprise) and thatattackers have a numerical advantage (a deduction based on theassumption that all members of a targeted hunting party havebeen spotted).If entirely accurate assessments could be made, weaponswould amplify the effects of both numerical superiority andsurprise, and the consequences of augmented lethality wouldaccrue to attackers. In other words, the decisive factor in thebalance of power between intruders and territory owners isdetection, not weaponry. (That asymmetrical detection outweighssuperiority in weapons and numbers and definitivelydetermines the outcome of territorial incursions is precisely thepoint Jacko and Topsy sought to make.) Moreover, the risks ofboth asymmetrical detection and assessment errors are intrinsicallygreater for intruders as they endeavor to move intounfamiliar territory in which hunters employing ambush techniquesmay well be operating from concealed positions. Thenecessity of movement in initiating an attack is itself a disadvantagewith respect to detection. Moreover, territory ownersinvariably sound the alarm if combat is joined. Other hunters inthe vicinity converge on the area and may potentially cut off theretreat of intruders. In sum, projectile weapons amplify thecasualties (and costs) of initiating attacks on neighbors as a resultof the intrinsic advantages enjoyed by defenders.Although the differences are striking, there also are a numberof notable similarities between chimpanzees and bonobos (onthe one hand) and the members of unsegmented foragingsocieties (on the other) in the realm of relations between groups.The potential for lethal intergroup violence is an ambientcondition of existence in both cases, and we can conclude thatthis potentiality has been an integral contextual feature ofhuman (hominid) evolution from the beginning of the Paleolithicperiod to the ethnographic present. Three responses to thiscondition are noted in both cases: (i) avoidance, (ii) positiveengagement in friendly relations with neighboring local groups,and (iii) aggression that may result in territorial gain or loss.Friendly relations facilitate full exploitation of the zone alongterritorial borders, whereas hostile relations lead to borderavoidance and underutilization of the food resources present inthis zone. Hostile relations are conditioned by resource competitionand also vary in intensity in accordance with the degree ofresource competition (other things being equal). The subjectsinvolved in acts of aggression against neighbors are nearly alwaysadult males who are the group members that most frequentlyexploit food resources located in the zone along territorialborders. Attacks are invariably opportunistic, in that the assessmentof a significant advantage (in numbers or in asymmetricaldetection or both) is always a precondition for initiating violencethat is lethal in intent. Nearly all attacks occur in close proximityto territorial borders. Attacks on base camps where groupmembers sleep are nonexistent among chimpanzees and rare tononexistent on the part of unsegmented foraging societies.Elaboration of the means of maintaining friendly relations andthe capacity for sharing access to resources with neighbors sethumans apart from chimpanzees and bonobos. Indeed, it mightbe said that the members of unsegmented foraging societiesincrease their fitness through ‘‘improved access to resources suchas food, females, or safety’’ (ref. 6, p. 12) by eschewing efforts toachieve intercommunity dominance in favor of egalitarian relationsof friendship, mutuality, and sharing. This course of actionis a wise adaptive choice because dominance is characteristicallyunattainable, and the only effective means of increasing territorysize is to fully utilize border zones. The capacity to maintainfriendly relations that allow for access to a neighbor’s territoryduring lean years is particularly important in environmentswhere there are localized year-to-year fluctuations in resourceavailability. These cooperative relations clearly played a key rolein the Upper Paleolithic expansion of human populations acrossthe globe into every environmental zone in which terrestrialmammals of any kind are capable of existing (9). Intergroupcooperation facilitates the rapid colonization of open environments.Under these circumstances, territory size is not a relevantvariable and fitness does not correlate with it. Fitness insteadcorrelates with the social group’s reproductive rate, which isprimarily a function of the ease of obtaining reproductivefemales from neighboring groups (based on a past history ofpositive relations).We may now turn to the archaeological record to ascertain theearliest known use of projectile weapons that kill at a distance.At present, that date has reliably been established as about400,000 years ago. Wooden spears 1.82–2.60 m in length that‘‘resemble modern javelins’’ have been recovered from a sitenear the Scho¨ningen brown coal mine in Germany.Found in association with stone tools and butcheredremains of more than 15 horses, the seven spears. . .strongly suggest that systematic hunting, involving foresight,planning, and appropriate technology, was part ofthe behavioral repertoire of premodern hominids. Theuse of sophisticated spears at these remote times necessitatesrewriting of many current theories on earlyhuman behavior and culture.Ref. 10, p. 8There is considerable continuity in the stone tools manufacturedby Homo erectus during the period from 1,000,000 to300,000 years ago. Temporal homogeneity in the stone componentof the tool kit provides grounds for suggesting that this samecontinuity applied to wooden implements as well. Woodenspears like those described above may then have been used 1million years ago. Large game made up a significant componentof the diet during this period, and it is reasonable to assume thatspears were used in game procurement and in driving offscavengers. Grahame Clark (11) provides a concise summary ofthe diet and probable tool kit of the H. erectus inhabitants ofZhoukoudian Cave in northern China, a site dated at 500,000–350,000 B.P.To judge from the animal remains associated with him,Peking man depended largely on venison, since twothirdsof them belong to two species of deer, namelyEuryceros pachyosteus and Pseudaxis grayi. His victimsalso included elephants, two kinds of rhinoceros, bison,water buffaloes, horses, camels, wild boars, roebucks,antelopes, and sheep, not to mention such carnivores assaber-toothed tigers, leopards, cave bears, and a hugehyena. How he managed to secure this varied selectionof game we can only speculate. No specialized projectileheadshave survived in the archaeological record, but tojudge from evidence from elsewhere, he would have hadavailable wooden spears with the tip hardened in fire,and it seems likely in view of the character of some of hisvictims that he would have used primitive pit traps.Ref. 11, p. 27The selective factors that favored coalitionary killing of neighborsmay have remained in play until as late as 1 million yearsago. The precise chronology of the persistence of these selectivefactors during the Lower Paleolithic remains an open questionat present. However, the development of the throwing spear,used in conjunction with ambush hunting techniques, ushered inan era in which the enhanced lethality of weaponry amplified thecosts of assessment errors, and the necessity of movement alsoplaced intruders at a comparative disadvantage with respect toboth detection and assessment. Moreover, asymmetrical detectionrather than a numerical imbalance of power determined theoutcome of hostile encounters. This reconfiguration of thedecisive factors in lethal conflict not only raised the stakes (orpotential costs) for would-be aggressors but also rendered thebenefit of intercommunity dominance unattainable. Becausesuperior numbers were not invariably decisive in encountersbetween hunting parties, an initial success would neither materiallyreduce the stakes for aggressors in subsequent attacks normake it possible to freely encroach on the territory of aneighboring group that had sustained a casualty. Under thesecircumstances, aggression resulted in stalemate and a conditionanalogous to a war of attrition rather than territorial gain.These developments marked a major turning point in theevolution of lethal intergroup violence and in the character ofinterrelations between neighboring groups. Although fitnesscontinued to be related to territory size (for food-limited populationsin occupied environments), selective circumstances nolonger favored aggression as a means of achieving territorialgain. Conflict avoidance and the development of intergrouprelations of friendship, mutuality, sharing, and cooperation werefavored instead. Intragroup cooperation was elaborated in conjunctionwith the teamwork entailed by large game hunting andwas further reinforced by mechanisms for sharing large animalsjointly killed by a hunting party. Development of these practicesprovided a template for establishing positive relations betweenneighboring social groups that could readily be realized whentwo bands temporarily camped and hunted together. Conflictavoidance dictated that groups move apart to alleviate resourcecompetition whenever possible. This response to crowding facilitatedmigration out of Africa into the temperate and subtropicalzones of Europe and Asia, vastly expanding the geographicdistribution of H. erectus. In other words, thedevelopment of the throwing spear altered the means of productionas well as the social relations of production, distribution,and consumption within groups in fundamental ways that alsotransformed intergroup relations and influenced subsequenthominid evolution.This period of Paleolithic warlessness, grounded in low populationdensity, an appreciation of the benefits of positiverelations with neighbors, and a healthy respect for their defensivecapabilities, lasted until the cultural development of segmentalforms of organization engendered the origin of war (7). Thisorganizational transformation facilitated the mobilization of alladult male group members and their participation in preplanneddawn raids on settlements in which the tactical advantages ofsurprise and numerical superiority could be brought to bear. Atthis juncture, the unit involved in combat is a raiding party (amilitary organization) rather than a hunting party (an economicorganization), and the location of combat shifts from the borderzone to the sleeping quarters at the core of a group’s territory.At the same time, the intrinsic military advantage shifts fromdefenders to attackers. All of the attackers are combatants,whereas less than half of those under attack are armed. Attackerscharacteristically inflict numerous casualties while suffering fewor none. This outcome is a consequence of weaponry amplifyingthe advantages of surprise and numerical superiority.The earliest conclusive archaeological evidence for attacks onsettlements is a Nubian cemetery (site 117) near the present-daytown of Jebel Sahaba in the Sudan dated at 12,000–14,000 B.P.(7, 12). War originated independently in other parts of the worldat dates as late as 4,000 B.P. (13). Otterbein argues thatagriculture was only able to develop initially at locations whereambushes, battles, and raids were absent (14).The evolution of lethal intergroup violence thus encompassesthree major periods: (i) the era of coalitionary killing, (ii) the eraof intrinsic defensive advantage, and (iii) the era of war. Anadvance in weapons technology (the javelin-like throwing spear)engenders the first transition, whereas an advance in militaryorganization and tactics produces the second. The decisivesignificance of these factors is expectable in light of what weknow from recorded military history. However, the duration ofthe first two eras extends over hundreds of thousands of years.The protracted character of these eras is consistent with the slowpace of technological and organizational change during thePaleolithic period.The main objective of this paper is to specify the major turningpoints in the evolution of lethal intergroup violence, to delineatethe periodization that results from this specification, and tobroadly characterize each era. The dating of these periods can beexpected to be refined as additional archaeological data come tolight. Moreover, the key transitions did not occur simultaneouslyin every world area so that regional, rather than global, chronologiesare required, especially with respect to the origin of war.I thank Kent Flannery for assistance in locating archaeological sourcesand Joyce Marcus for her useful comments on an earlier draft.

Notes

#E-mail: rck@umich.edu.†This point can be further elaborated. It is possible that adding females without increasingterritory could augment fitness, but this fitness enhancement would transpire only if foodresources were more than adequate. However, adding territory would not augmentfitness under such conditions. In other words, Wrangham’s argument presupposes thatchimpanzees are a food-limited (rather than disease-limited) population, because intercommunitydominance would otherwise convey no selective advantage. See ref. 15 for afurther discussion of the interrelationship between the food-limited status of a populationand the adaptive significance of territorial gain.

‡Unsegmented societies are defined by eight organizational criteria (7). In brief, the socialorganization of unsegmented societies is limited to the bare minimum: these societiesmanifest only those social groups that are cultural universals (present in every society) andnothing beyond this minimal set. Local groups are comprised of independent families(including a man, one or more spouses, and their children). These communities areautonomous, i.e., there is no level of organization beyond the local group. Egocentricbilateral kin networks or kindreds link individuals to relatives in other communities. Thus,family, local group, kindred, neighborhood, and language group (‘‘one-talks’’) arepresent. Descent groups, characteristic of segmental organization, are absent.

§The armed conflict that occurs between hunting parties in unsegmented societies differsfrom warfare in that only the perpetrator of an offense is targeted. Likewise, a homicidein an unsegmented society may result in an attempt to execute the perpetrator. Capitalpunishment and warfare share a number of features (ref. 7, p. 7) but differ in one criticalrespect: in war, social substitution governs the targeting of individuals for lethal violence.These two forms of lethal violence can readily be distinguished by a simple exampleillustrating the choices that confront a member of a typical hunting and gathering bandcomposed of a half-dozen related families. If a member of a neighboring group kills anindividual’s sister, there are two alternative retaliatory responses: the aggrieved brotherof the woman may kill the killer, or he may kill the killer’s sister (or parent or child or anymember of the killer’s social group). In unsegmented foraging societies, only the firstcourse of action is considered morally justified. The second course of action would not beconsidered to resolve the situation in any way and indeed would constitute an independentevent. Although some kinsmen and coresidents might potentially assist an individualin carrying out a capital punishment execution, they would not assist in the second courseof action nor dependably come to the defense of the perpetrator of a homicide. Thetransition from capital punishment to war thus requires development of the culturalconcepts of injury to the group and group responsibility for the inflection of injury (7).These concepts render any member of the killer’s collectivity a legitimate target for bloodvengeance and, at the same time, mobilize a vengeance party.The development of segmental forms of organization leads to revenge-based raidinginitiated in response to an initial homicide stemming from a commonplace interpersonalconflict. Such conflicts are characteristically unrelated to resource scarcity and are notdensity dependent. As a result, the correlation between the frequency of lethal intergroupviolence and the degree of resource scarcity that obtains among unsegmented societies isnot applicable to segmental societies (7).¶In unsegmented societies, it is not uncommon for living parents to give a child to kin orfriends of another local group to be adopted and raised by them. Among the Andamanese,nearly all children older than 7 years of age are raised by adoptive parents (7). Thiscultural practice is obviously conducive to mutual nonaggression between local groupslinked by such child transfers.This generalization is based on detailed analysis (7) of the eight unsegmented foragingsocieties in Ross’s (16) sample of 93 societies [which is a half sample of Murdock’s andWhite’s (17) Standard Cross-Cultural Sample]. Attacks on settlements per se are notreported for the Mbuti, Semang, Copper Eskimo, !Kung, Yahgan, and Slave. The Slave weresubject to such attacks by the segmental Cree but did not carry them out. The Ingalik werelikewise subject to raids on their settlements by their segmental neighbors (the Koyukonand Kolchan) in the 1840s, and they responded in kind. However the Ingalik did not carryout raids against the settlements of fellow tribesmen or those of their unsegmentedneighbors, the Kuskowagamuit. Among the !Kung and Yahgan, the family of a homicidevictim may go to the settlement of the perpetrator and attempt a capital punishmentexecution, but only the perpetrator is targeted, so this endeavor does not constitute anattack on a settlement per se. Attacks on settlements are reported for the Andamanese,who represent a transitional case with respect to the origin of war. However, no suchattacks are recorded in colonial records that cover the period from 1863 to 1899. Theserecords contain detailed accounts of numerous border conflicts during this early contactperiod. In all, these data indicate that, for unsegmented societies, attacks on settlementsmay be characterized as rare to nonexistent. Events that occur less than once a generationare considered rare in the comparative cross-cultural study of lethal violence. Such eventsare generally not within the life experience of the members of these societies.